1) Macs do tend to run hotter than PCs, and this could decrease HDD lifespan.
2) We've seen in the past that OS-specific power management settings can be detrimental to hard drives, as with this famous Linux bug: https://bugs.launchpad.net/ubuntu/+bug/104535 Although I doubt it's actually the case, it's possible in principle for drives to wear differently under OS X than under Windows.
Google found that temperature didn't much matter according to their published research: http://labs.google.com/papers/disk_failures.pdf
> In the lower and middle temperature ranges, higher temperatures are not associated with higher failure rates. This is a fairly surprising result, which could indicate that data center or server designers have more freedom than previously thought when setting operating temperatures for equipment that contains disk drives.
Non-PDF summary/discussion: http://storagemojo.com/2007/02/19/googles-disk-failure-exper...